It has long been recognized that analyzing fecal specimens for parasite ova by microscopy is a simple and effective method for identifying parasites afflicting a patient. This method is routinely used in clinical and veterinary laboratories around the world to identify specific parasites in fecal specimens from animals and humans so that the patient may be properly treated for the affliction.
There are a variety of laboratory techniques in common use to detect the presence of ova in a fecal sample. The simplest of these is the direct smear technique in which a small sample of patient feces is mixed with saline and “smeared” across the surface of a microscope slide. A coverslip is placed over the smear and the specimen is examined microscopically for parasite ova. This technique is rarely used in modern laboratories because the presence of debris in the fecal sample makes direct examination extremely difficult and prone to error. Also, the small sample size used makes it likely that a low population of parasites, such as during the early stages of an infestation, may not be detected.
For many years the preferred technique, in several variations, has been the use of a float-or-sink process in which a reagent liquid of a density between that of the ova and that of fecal matter is vigorously mixed into the fecal specimen to allow ova contained within the feces to be released to the liquid, and the ova then allowed to separate by floatation from the fecal debris. The ova, having floated to the top of the liquid, are then transferred to a microscope slide, such as by touching a coverslip to the surface of the liquid and placing the coverslip onto a microscope slide. Under the microscope, the type of ova and therefore the specific parasites present in the sample can be identified, and the seriousness of the infestation can be determined by counting and recording the number of each type of ova. This prior art process has been improved over the years, but still is not optimized and suffers from several limitations including the risk of exposure of laboratory personnel to potentially dangerous pathogens, complexity, unpleasant odor and also a degree of unreliability or inaccuracy.
Early improvements to this procedure were a) the prefiltration of the feces and floatation mixture, typically through a strainer, to remove clumps and undigested vegetable matter which may be contained in the fecal specimen and which would float to the surface of the liquid along with the ova, and b) the centrifugation of the prefiltered mixture to accelerate the process and provide a sharper separation of ova and fecal debris. While this latter technique proved to be more accurate and reliable, the multiple transfer steps involved and the high potential for spills and aerosol generation limited the acceptance of the technique as a routine laboratory procedure.
From the 1970s onward, a number of low-cost stand-alone devices were introduced which combined sample collection, filtration of debris, and ova collection in a single disposable unit. The first of these devices, called the FECALYZER, consists of a container and a combination sample collection and straining part. In operation, the container is filled with a floatation reagent, the sample collector and strainer is used to collect a measured amount of feces and to effect mixing of the sample and floatation liquid in the container to release ova from the fecal sample and to allow passage of the ova through the integral strainer while retaining vegetable matter and other fecal debris in the lower portion of the device. A coverslip is placed upon the opening of the device at the liquid surface to receive the floating ova. While the FECALYZER and other similar devices such as the OVASSAY are widely used today, particularly in companion animal veterinary practices, the devices of this technique have several key deficiencies including a) the time required for complete floatation of ova to the surface of the liquid can be excessive thereby making it impossible to provide a diagnosis during a typical one-half hour patient appointment and b) without the benefit of prefiltration or pre-separation by centrifugation, the straining method of these devices under natural gravity may trap a portion of the ova thereby contributing to inaccuracy of the diagnosis.
The accuracy and sensitivity of veterinary fecal exams have recently come under close scrutiny because many common parasites found in companion animals can be transmitted from pets to owners. As a result, a number of professional veterinary societies have studied the accuracy and repeatability of various techniques for fecal parasite analysis and have universally concluded that only the centrifugal floatation method can produce the accuracy and sensitivity necessary to protect both pets and their owners and that the widely-used FECALYZER and similar devices are suboptimal for the procedure.
In the standard method for centrifugal floatation using a swinging bucket type laboratory centrifuge device, the fecal sample is placed in a tube, the tube is filled with a floatation fluid to form a meniscus, and a cover slip placed on top of the tube. Care is taken to avoid the trapping of air bubbles between the fluid and the underside of the coverslip. The tube is placed in a swinging bucket centrifuge and spun for an appropriate time, about ten minutes, during which fecal matter and debris that have a higher density than the floatation fluid are forced to the bottom of the tube, while the buoyant ova are forced to the surface of the fluid and adhere to the coverslip. After the spin cycle, the coverslip is removed and transferred to a microscope slide for analysis using standard microscopic methods. This centrifugal method has several drawbacks. First, swinging bucket centrifuges are costly and large, and are rarely found in veterinary laboratories and other environments where fecal floatations are performed. Moreover, the tube is subject to spillover when it is filled with the floatation fluid to form a meniscus. Also, the coverslip can be flung off if the centrifuge rotor accelerates or decelerates at a high rate.
In the standard method for centrifugal floatation using a fixed angle type centrifuge, of the kind commonly found in clinics and veterinary laboratories, the tube is restrained at an acute angle with respect to the vertical axis of rotation. Thus, the tube cannot be filled to the top to form a meniscus prior to centrifugation, and consequently a coverslip cannot be placed on the tube before the centrifugation step. The meniscus must be formed by adding fluid to the tube after centrifugation, which disturbs the ova floating on the surface. The tube must rest for about ten minutes after the fluid addition to allow the ova to ascend to the fluid surface and adhere to the coverslip.